The present invention relates to an electrostatographic apparatus typified by an electrostatic copying machine.
Such a copying machine comprises an electrostatic drum which is rotated at constant speed, electrostatically charged and radiated with a light image of an original document to form an electrostatic image thereof. A toner substance is applied to the drum to develop the electrostatic image into a toner image. A copy sheet is moved into engagement with the drum and an electrostatic charge applied thereto to transfer the toner image to the copy sheet. The toner image is fixed to the copy sheet to provide a permanent reproduction of the original document.
Problems have remained heretofore unsolved regarding positively separating the copy sheet from the drum after toner image transfer while preventing the toner image from being degraded. A prior art attempt to overcome these problems involves applying an alternating corona discharge potential to the copy sheet to dissipate the effect of the transfer charge and remove the electrostatic attractive force between the copy sheet and the drum. Such an expedient is also desirable since dissipation of the electrostatic charge on the copy sheet prevents image degradation due to localized discharge if the copy sheet contacts metal parts of the copy machine after removal from the drum and before toner image fixing. However, application of alternating corona discharge tends to cause the copy sheet to separate from the drum during the transfer operation, especially where the transfer operation takes place at a lower portion of the drum, due to the weight of the copy sheet. This results in missing image areas in the copy.
In an attempt to overcome these difficulties it has been proposed to add a D.C. bias to the A.C. corona discharge. The bias voltage has a polarity which is the same as that of the toner substance for separating the leading edge of the copy sheet from the drum. Then, the polarity of the bias voltage is reversed to reduce the discharge effect on the copy sheet and prevent the remaining portion thereof from separating from the drum during the transfer step.
This expedient is not satisfactory either since a separate D.C. bias voltage source must be provided and the toner image at the leading edge portion of the copy sheet differs from the toner image on the remainder of the copy sheet.